It is desirable to be able to perform time-domain reflectometry analysis of network cables, for example Ethernet/IEEE 802.3 coaxial cable systems, to assist in the isolation and repair of cable faults and other network problems. Such determination will typically employ a time-domain reflectometer which transmits a series of pulses into the network cabling and determines the existence and location of cabling faults based on the reflections received in response to the pulses and the amount of time between the pulse transmission and the received reflection.
The time-domain reflectometer is also useful in measuring exact cable length which may be desirable in certain network applications.
The International Standards Organization (ISO) has set forth a standard reference model of an open systems interconnection (OSI) to define the way that participating network nodes interact for exchanging information. This standard provides for seven layers of protocol, wherein the first layer is the physical communication layer wherein nodes of a network are physically interconnected in some manner. All other communications beyond the first layer are not physically connected but appear to be to the user.
In an Ethernet system, access to the network is provided via a carrier sense multiple access with collision detection (CSMA/CD) standard. In such a system, a station that wishes to transmit will first determine whether the network is currently in use (carrier sense) and if the network is not in use, the station will begin transmitting. Also the CSMA/CD system includes collision detection wherein if the station begins transmitting and soon thereafter detects a collision between its transmission and the transmission of another station, which may not have been readily apparent prior to transmitting because of propagation delays along the network, the station detecting the collision will take corrective action. In the case of an Ethernet system, this corrective action consists of sending a jam pattern which the other transmitting station, which was collided with, will recognize to indicate a collision did occur. Retransmission is then accomplished according to a "back-off algorithm" wherein each of these two or more stations involved in the collision will wait a random amount of time before retransmitting. Thus, the Ethernet system makes use of collision between station transmissions to efficiently regulate access to the network by various nodes.
When performing time-domain reflectometry on an active network, however, the pulses which are generated by the time-domain reflectometer are of such short duration as to go undetected by any of the carrier sense/collision detection systems that are designed in accordance with the international standards for Ethernet, for example ANSI/IEEE Standard 802.3. Therefore, when a collision does occur between a time-domain reflectometry pulse and data transmitted by a station on the network, the transmitting station does not detect it, and bit errors will be generated in the station's transmitted data by the time-domain reflectometry pulses. The error will likely be eventually detected at a higher layer in the protocol scheme; however, the higher in the protocol layers that error detection occurs, the longer it takes for a timeout to be completed based on the bit error that was caused by the collision with the time-domain reflectometry pulse. For example, in an Ethernet system, level 3 and level 4 timeouts are in the area of two to three seconds long. Higher level timeouts can be up to one minute long or more. Such delay is noticeable to network users and can lead to user dissatisfaction with the network.
In order to avoid problems with data corruption from time-domain reflectometry pulses, heretofore it has been necessary to bring the network down or inform all users to stop using the network and observe a network silence period while network diagnostics (particularly time-domain reflectometry) were performed. Such solutions are unsatisfactory as they tend to interfere with user confidence in the network. Also, network silence may not be observed by all users. It is desirable to perform testing and measurement on active networks since some network problems may only arise when the network is active and problems may be difficult to solve if they cannot be detected and observed by a technician as they are occurring.